1. Field of the Invention
The present invention relates to an aqueous pigment dispersion composition for electro-deposition color coating which is used to color an aluminum or aluminum alloy oxide film formed on a product, such as a building material or a vehicle wheel, made of aluminum or an aluminum alloy for use in a wide range of applications. More specifically, the invention relates to an aqueous pigment dispersion composition for electro-deposition color coating which is used as a pigment component of an electro-deposition coating liquid for formation of a matte coating film on an aluminum or aluminum alloy oxide film and provides for less lustrous, less transparent and highly weather-resistant coating properties.
2. Description of the Prior Art
One conventional method for forming an electro-deposition matte coating film on an aluminum or aluminum alloy oxide film includes the steps of immersing a lustrous electro-deposition coating film in an acid bath before the coating film cures, and promoting the curing of the coating surface. In another method, an emulsion of a non-polar resin such as polyethylene is blended with a lustrous electro-deposition coating liquid, and undulations are formed on the surface of the resulting coating film by utilizing the immiscibility between a base resin component and a wax component of the coating liquid and the vaporization of the wax component under heating. In this method, the fusion and thermal fluidity of the coating film at baking can be suppressed by preliminarily crosslinking emulsion particles (Coloring Materials, 66(11), 671-676 (1993)).
In these methods, however, consideration is given to a coating resin, but not to a pigment component, to provide for the matte coating property. It is essential to impart the matte (or less lustrous) coating property as well as an opaque (or less transparent) coating property to a coating film on an aluminum or aluminum alloy oxide film, because the surface of the aluminum or aluminum alloy oxide film should be concealed. Titanium oxide is conventionally used to provide for the opaque coating property. However, the use of titanium oxide is not preferred because of a poor weatherability, often resulting in chalking. Particularly, where a coating liquid containing titanium oxide is used for exterior materials such as aluminum building materials and aluminum wheels, the aforesaid problems are critical.